Recently, display technologies using various methods such as a plasma display panel (PDP), a liquid crystal display (LCD) and the like that replace a known Braun tube in accordance with the development of optical technologies are suggested and marketed, and a high level of characteristics are required for the polymer material for the display. For example, in the case of the liquid crystal display, according to the development toward the thin film, the lightness, and enlargement of the screen area, the wide viewing angle, the high contrast, the suppression of change in image color tone according to the viewing angle and the uniformity of the screen display are particularly considered as important problems.
Accordingly, various polymer films are used in a polarizing film, a retardation film, a plastic substrate, a light guide plate and the like.
There are various modes such as Double Domain TN (twisted nematic), STN (super twisted nematic), ASM (axially symmetric aligned microcell), OCB (optically compensated blend), VA (vertical alignment), MVA (multidomain VA), SE (surrounding electrode), PVA (patterned VA), IPS (in-plane switching), and FFS (fringe-field switching) modes in a liquid crystal display. Each mode has an intrinsic liquid crystal arrangement and optical anisotropy. Therefore, a compensation film of optical anisotropy corresponding to each mode is required in order to compensate retardation due to optical anisotropy of the liquid crystal mode.
An IPS mode of liquid crystal panel has a characteristic of wide viewing angle as compared to a known TN mode, and a STN mode. However, in the IPS mode, in a perpendicular direction of the panel, an almost complete black display can be implemented, but in the case when the panel is observed in a misaligned direction to the perpendicular direction, in a misaligned direction to an optical axis direction of the polarizing plate that is disposed at upper and lower parts of the liquid crystal cell, there is a problem in that light leakage inevitably occurs because of a characteristic of the polarizing plate. In addition, the panel size is being currently increased, and a color shift that was not considered as a problem in the related art becomes an issue.
In order to solve this, in the case of the IPS mode, since a liquid crystal that has a positive dielectric anisotropy is filled between the polarizing plates, an in-plane refractive index is more largely aligned than a thickness refractive index, such that a +C-plate type anisotropy film that has a thickness positive retardation is required as an optical compensation film.
Among the polymer materials, when the polymer chain is stretched and aligned, polycarbonate (PC) shows the optical anisotropy in which the in-plane refractive index is increased and has a negative thickness retardation, such that it is not desirable to apply it alone to an IPS mode, and polycarbonate is used as a (−) C-plate that is an optical film for compensating a viewing angle of a VA mode of liquid crystal display.
In general, a compensation film for IPS compensates the viewing angle by uniaxially stretching COP and coating a nematic liquid crystal that is a +C plate. However, in this case, since physical properties are excellent but birefringence of the liquid crystal is very high, even if the alignment and the coating thickness of the liquid crystal are slightly changed, the entire retardation of the compensation film is largely changed, such that it is difficult to control the retardation in the case of a thin film. In addition, since a manufacturing cost is increased because of a high cost of liquid crystal, it is disadvantageous in that it is difficult to commercialize it, and in the case of liquid crystal coating, since Tg is basically about 80C, there is a limit in use as a material for the current polarizing plate that requires high durability.
In addition, in the case of a known product using the liquid crystal, since an upward compensation characteristic is excellent but a downward compensation characteristic is relatively low, it is not suitable to apply it to a material that requires a characteristic of entirely sufficient compensation region.
Meanwhile, as an example of using a stretched acryl film, there are a multilayered structure (Korean Unexamined Patent Application Publication No. 2006-0054391), and an arylate coating layer (Korean Unexamined Patent Application Publication No. 2007-0003388), but since they are applied to a VA (vertical alignment) mode of liquid crystal display and application ranges of retardation values are different from each other, it could not be applied to an IPS (in-plane switching) mode of liquid crystal display.
As a patent relating to a compensation film for IPS, there are Korean Unexamined Patent Application Publication Nos. 2008-0004720 and 2006-0047433, in which a MMA-Styrene-MAH copolymer film is manufactured by uniaxial and biaxial stretching, a +C plate is manufactured, and a manufacturing cost can be reduced. However, in the case when a Nz (Rth/Rin) value that is a ratio of a thickness retardation value to an in-plane retardation value is larger than or equal to 1, a viewing angle compensation is reduced as compared to the case when the above-mentioned liquid crystal is coated.
In practice, it is very difficult and impossible to set the Nz (Rth/Rin) value that is the ratio of the thickness retardation value to the in-plane retardation value to be less than 1 by using only the stretched film.